SourceTree, AFAIK is NOT open-source. It's closed-source freeware.
The Atlassian EULA is here:
https://www.atlassian.com/legal/customer-agreement
The deprecated SourceTree Corporate EULA is here:
https://confluence.atlassian.com/sourcetreekb/sourcetree-license-for-corporate-usage-388404078.html
It's not Open Source at all.
Annoyingly, I can't find the licence terms, but I do know you get them if you register and ask for a (free) licence.
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